Electrochemical energy conversion devices include fuel cell systems as well as hydrogen generators and other electrolysers, such as for co-electrolysing water and CO2.
Fuel cells convert gaseous fuels (such as hydrogen, natural gas and gasified coal) via an electrochemical process directly into electricity. A fuel cell continuously produces power when supplied with fuel and oxidant, normally air. A typical fuel cell consists of an electrolyte (ionic conductor, H+, O2−, CO32− etc.) in contact with two electrodes (mainly electronic conductors). On shorting the cell through an external load, fuel oxidises at the negative electrode resulting in the release of electrons which flow through the external load and reduce oxygen at the positive electrode. The charge flow in the external circuit is balanced by ionic current flows within the electrolyte. Thus, at the positive electrode oxygen from the air or other oxidant is dissociated and converted to oxygen ions which migrate through the electrolyte material and react with the fuel at the negative electrode/electrolyte interface. The voltage from a single cell under load conditions is in the vicinity of 0.6 to 1.0 V DC, and current densities in the range of 100 to 1000 mAcm−2 can be achieved. In addition to the electricity, water is a product of the fuel cell reaction. Hydrogen generators and other electrolysers may be considered as fuel cell systems operating in reverse. Thus, a hydrogen generator produces hydrogen and oxygen when electricity and water are applied to the electrochemical cell.
A fuel cell system capable of producing electricity may be designed to run in reverse in order to produce hydrogen, for example producing electricity during the day and hydrogen at night, with the hydrogen optionally being stored for use the next day to produce more electricity. However, it may be advantageous from the efficiency perspective to design separate fuel cell systems and hydrogen generators. While the invention is concerned with electrochemical energy conversion devices generally, for convenience only it will be described hereinafter primarily with reference to electricity generating fuel cell systems and cells for them.
Several different types of fuel cells have been proposed. Amongst these, solid oxide fuel cell systems (SOFC) are regarded as the most efficient and versatile power generation system, in particular for dispersed power generation, with low pollution, high efficiency, high power density and fuel flexibility, and the invention is particularly concerned with solid oxide electrochemical energy conversion cells and with devices using them. Numerous SOFC configurations are under development, including tubular, monolithic and planar designs, and are now in production. The planar or flat plate design is perhaps the most widely investigated and now in commercial use, and the invention is particularly concerned in one aspect with electrochemical energy conversion devices comprising a stack of such solid oxide electrochemical cells. However, in another aspect, the invention also extends to solid oxide electrochemical energy conversion cells generally, that is it is concerned with tubular cells and monolithic cells, as well as with planar cells.
For convenience only, the invention will be further described solely with respect to planar or flat plate design solid oxide electrochemical energy conversion cells, and devices using them. In these devices, individual planar SOFCs comprising electrolyte/electrode laminates alternate with gas separators, called interconnects when the gas separators convey electricity from one SOFC to the next, to form multi-cell units or stacks. Gas flow paths are provided between the gas separators and respective electrodes of the SOFCs, for example by providing gas flow channels in the gas separators, and the gas separators maintain separation between the gases on each side. Apart from having good mechanical and thermal properties, as well as good electrical properties in the case of interconnects and good electrochemical properties in the case of the fuel cells themselves, the individual fuel cell device components must be stable in demanding fuel cell operating environments. SOFCs operate in the vicinity of 600° C.-1000° C. and, for devices using them to be economical, typical lifetimes of 5-6 years or more of continuous operation are desired. Thus, long term stability of the various device components is essential. Only a few materials fulfil all the requirements. In general, the high operating temperature of the SOFCs, the multi-component nature of the devices and the required life expectancy of several years severely restricts the choice of materials for the fuel cells, gas separators and other components such as seals, spacer plates and the like.
A variety of different materials have been proposed for SOFC gas separators, including ceramic, cermet and alloys. For electrically conductive gas separators, that is interconnects, metallic materials have the advantage generally of high electrical and thermal conductivities and of being easier to fabricate. However, stability in a fuel cell environment, that is high temperatures in both reducing and oxidising atmospheres, limits the number of available metals that can be used in interconnects. Most high temperature oxidation resistant alloys have some kind of built-in protection mechanism, usually forming oxidation resistant surface layers. Metallic materials commonly proposed for high temperature applications include, usually as alloys, Cr, Al and Si, all of which form protective layers. For the material to be useful as an interconnect in SOFC devices, any protective layer which may be formed by the material in use must be at least a reasonable electronic conductor. However, oxides of Al and Si are poor conductors. Therefore, alloys which appear most suitable for use as metallic interconnects in SOFCs, whether in cermet or alloy form, contain Cr in varying quantities.
Cr containing alloys form a layer of Cr2O3 at the external surface which provides oxidation resistance to the alloy. The formation of a Cr2O3 layer for most electrical applications is not a problem as it has acceptable electrical conductivity. However, for SOFC applications, a major problem is the high vapour pressure and therefore evaporation of oxides and oxyhydroxides of Cr Cr6+) on the positive electrode side of the fuel cell at the high operating temperatures. At high temperatures, oxides and oxyhydroxides of Cr (Cr6+) are stable only in the gas phase and have been found to react with positive electrode materials leading to the formation of new phases such as chromates, which destroy the electrode material and make it electrically resistive, as well as to deposits of Cr2O3 on the electrolyte. These reactions very quickly reduce electrode activity to the oxygen reduction reaction at and adjacent the positive electrode/electrolyte interface, and thereby considerably degrade the electrochemical performance of the cell.
It has been attempted to alleviate this problem of degraded electrochemical performance by coating the positive electrode side of the interconnect with a perovskite barrier layer such as strontium-doped lanthanum manganite (LaMnO3) (LSM), which may also be the material of the positive electrode, but while short term performance was maintained there continued to be an unacceptable long term degradation in performance.
The problem of degradation due to evaporation of oxides and oxyhydroxides of Cr from chromium-containing materials on the positive electrode side of the fuel cell was greatly relieved by the invention described in the applicant's WO96/28855, that is forming a self-repairing coating on the positive electrode side of a chromium-containing interconnect, the coating comprising an oxide surface layer comprising at least one metal M selected from the group Mn, Fe, Co and Ni and a M, Cr spinel layer intermediate the chromium-containing substrate of the interconnect and the oxide surface layer. Such a coating may also be formed on other chromium-containing heat resistant steel surfaces that are on the positive electrode side of the plant. However, it remains a challenge to ensure the coating remains fully dense to prevent the release of the chromium species in the demanding fuel cell operating conditions.
Other solutions have also been proposed for alleviating the degradation in fuel cell performance due to evaporation of oxides and oxyhydroxides of Cr on the positive electrode side of the fuel cell. For example, a low (or no) chromium steel is proposed in the applicant's WO00/75389, in which an alumina coating is formed on oxidation of the surface rather than chromium oxide. However, due to the low electrical conductivity of alumina, this heat resistant steel composition is not suitable for gas separators that are intended to act as interconnects conducting electricity from one side to the other.
In a further effort to limit the problem of degradation due to evaporation of oxides and oxyhydroxides of Cr on the positive electrode side of the fuel cell, it has been proposed to introduce another layer (referred to hereinafter as “shield layer”) on the positive electrode layer to absorb chromium before it reaches the positive electrode layer.
Positive electrode materials for SOFCs are generally perovskites or oxides having perovskite-type structures (refined to herein as “perovskites”), such as lanthanum strontium manganite or LSM (La1-xSrxMnO3-δ), lanthanum strontium cobaltite or LSCo (La1-xSrxCoO3-δ), lanthanum strontium ferrite or LSF (La1-xSrxFeO3-δ), La1-xSrxCO1-yFeyO3-δ (LSCF), LaNixFe1-xO3-δ (LNF), and Ba1-xSrxCo1-yFeyO3-δ (BSCF) where 0≦δ<1 depending on the dopant. Other examples include SmxSr1-xCoO3-δ (SSC), LaxSr1-xMnyCo1-yO3-δ (LSMC), PrxSr1-xFeO3-δ (PSF), SrxCe1-xFeyNi1-yO3-δ (SCFN), SrxCe1-xFeyCo1-yO3-δ, PrxCe1-xCOyFe1-yO3-δ and PrxCe1-xCoyMn1-yO3-δ. In the strontium-containing perovskites, for example, the strontium is provided as a doping agent that is bound into the perovskite structure.
The aforementioned shield materials proposed to date have been perovskites, for example having a similar composition to the positive electrode layer but more reactive with chromium than the positive electrode material in order to absorb it before it reaches and reacts with the positive electrode layer. In one example where the positive electrode material is LSM the shield layer material is LSCo (La1-xSrxCoO3-δ), but other materials are possible.
Some barrier materials are proposed in the paper by Thomas Franco et al “Diffusion and Protecting Barrier Layers in a Substrate Supported SOFC Concept”, E-Proceedings of the 7th European Fuel Cell Forum, Lucerne (2006), P0802-051. This paper also sets out additional details on the reactions occurring.
Even with these advancements, it is found that degradation of fuel cell performance remains a problem. This has led to extensive further investigations by the applicant as to the causes, from which additional positive electrode material poisons have been identified.
As a result of these investigations the applicant has found that sulphur poisons the positive electrode of an SOFC in much the same way as chromium, by forming sulphate crystals with components of the electrode material, such as strontium, a d possibly destroying the chemical structure of the electrode material. It has been found that the sulphur may be derived from the oxidant supply (generally air), usually in the form of SO2, or from elsewhere in the system, for example in the glass seals used to seal the SOFCs and gas separators together or elsewhere upstream of the positive electrode-side chamber, where the sulphur may be present as an impurity and appear as SO2 or SO3.
The further investigations have also shown that boron can act in the same way as chromium and sulphur to poison the positive electrode material in the conditions of use. Boron may be present in the system as a compound of the glass seals, but may also be present in other components of the fuel cell system exposed to the oxidant.
It is believed that other element present in the system components, or in the oxidant supply, whether as impurities or otherwise, may also be reacting with components of the positive electrode material and poisoning the material. Possible examples of these elements include silicon.
Alleviating reactions with the positive electrode material by poisons in the system in use of an electrochemical energy conversion cell, and therefore alleviating cell performance degradation, is an aim of the invention described and claimed in a co-pending PCT patent application filed by the applicant concurrently herewith and claiming priority from the priority applications, entitled “Electrochemical Energy Conversion Devices and Cells, and Positive Electrode-Side Materials for them”, the contents of which are incorporated herein by reference.
However, the applicant's further investigations into the causes of fuel cell performance degradation has revealed that in addition to poisoning of the positive electrode material, the negative electrode side also suffers from performance degradation.
SOFC negative electrode materials are generally nickel based, most commonly Ni/YSZ cermets. Other nickel cermets being used as negative electrode materials include Ni/GDC (Ni/gadolinium doped ceria), Ni/SDC (Ni/samarium doped ceria), Ni/ScSZ (Ni/scandiastabilised zirconia) and Ni/ScCeSZ (Ni/scandia ceria stabilised zirconia). Pt, Rh and Ru have all been used in place of nickel in cermet negative electrode materials, but these metals are considerably more expensive than nickel and therefore much less common.
It is well known that sulphur reacts with nickel in negative electrode materials under SOFC operating conditions to degrade the performance of the electrode, and for this reason sulphur is commonly removed from SOFC fuel sources. However, the applicant's further investigations have led to a belief that, even if sulphur is removed from the fuel source, sulphur continues to degrade the negative electrode material. This is believed to be as a result of residual sulphur in the fuel or as a result of sulphur from elsewhere in the system, for example in the glass seals used to seal the SOFCs and gas separator or elsewhere upstream of the negative electrode, where the sulphur may be present as an impurity. Some of the reasons for degradation of the negative electrode material performance due to sulphur are believed to be: at very low sulphur levels, for example as low as 1 ppm in the gas stream, the electrode material can degrade due to surface adsorption of the sulphur on the nickel; at higher levels of sulphur, Ni—S alloys are formed; and at even higher levels of sulphur, nickel sulphides form.
The effect on SOFC anodic performance of hydrogen and hydrocarbon fuels contaminated with up to 50 ppm wet H2S was investigated by Limin Liu et al, in the paper “Sulfur Tolerance Improvement of Ni-YSZ Anode by Alkaline Earth Metal Oxide BaO for Solid Oxide Fuel Cells”, Electrochemistry Communications 19 (2012) 63-66. In the paper it is reported that BaO infiltrated throughout the functional anode layer at a level of about 5 wt % was found to enhance the sulphur tolerance ability of the Ni—YSZ anode over the test period of 27 hours. It was concluded that water played a very crucial role in this, and that this may result from the good water dissociative absorption ability of BaO.
The applicant's further investigations on the negative electrode side have also identified that boron and phosphorus species from seals and other components of the device may be entering the atmosphere in the negative electrode-side chamber and leading to performance degradation in some way. In the case of boron at least this appears to be by promoting grain growth in the nickel or other metal of the electrode material. The phosphorus species may be reacting with the nickel and poisoning it.
Other species that have been found to be detrimental to the negative electrode-side performance, possibly as a result of reacting with and thereby poisoning the nickel, are chlorine, siloxane and selenium. These may be present on the negative electrode side as impurities, for example, in the fuel gas or the glass used for the seals.
Another problem identified on the negative electrode side is the unintended ongoing sintering of nickel in porous layers in the negative electrode-side chamber, particularly but not only in the negative electrode-side structure of the electrochemical cell, including the negative electrode material. This sintering leads to a loss of surface area in the porous layer or layers and a decrease of the triple phase boundary area of the electrode layer, resulting in degradation in electrochemical performance.
It is clear that it would be highly desirable to alleviate long-term degradation of cell performance on the negative electrode side in use of an electrochemical energy conversion cell.